Yankee dryers have been used for many years to dry wet paper webs during the paper making process. Typically, the wet web is delivered to and pressed into engagement with the rotating dryer drum with the outer cylindrical surface of the dryer drum transporting the wet paper web through a dryer hood associated with the dryer drum. Both the dryer drum and the interior of the hood are heated to contribute to the drying operation.
It is known that paper making machines operators often use more makeup air than is actually required to carry away the water vapor within the interior of a yankee dryer hood. This significantly adds to cost of manufacture. The key indicator for economical operation is humidity of the exhaust air stream from the yankee hood. This measurement is very difficult to obtain because of the high temperatures of yankee exhaust air. Thus, the operator will usually err on the side of providing too much makeup air to ensure proper web drying.
Several commercial instruments, utilizing a variety of principles, have been tried in the past and, for a variety of reasons, none have proved satisfactory over time. Prior art techniques that have been tried or at least suggested are:
Insitu IR absorption PA1 Sampling IR absorption PA1 Insitu UV absorption PA1 Insitu oxygen analyzer PA1 Optical sampling dew pointer PA1 Capacitance sampling dew pointer PA1 Resonant piezoelectric sampling dew pointer PA1 Heat flow sampling dew pointer PA1 Sampling fluidic oscillator (density change with humidity) PA1 Hot wire anemometer (heat transfer changes with humidity) PA1 Insitu wet/dry bulb PA1 Sampling wet-dry bulb
A search directed to the present invention located the following U.S. Pat. Nos.: 1,894,172, issued Jan. 10, 1933, U.S. Pat. No. 2,316,624, issued Apr. 13, 1943, U.S. Pat. No. 3,037,375, issued Jun. 5, 1962, U.S. Pat. No. 3,265,301, issued Aug. 9, 1966, U.S. Pat. No. 3,532,270, issued Oct. 6, 1970, U.S. Pat. No. 3,665,748, issued May 30, 1972, U.S. Pat. No. 4,507,875, issued Apr. 2, 1985, U.S. Pat. No. 4,597,285, issued Jul. 1, 1986, U.S. Pat. No. 4,809,537, issued Mar. 7, 1989, U.S. Pat. No. 2,166,379, issued Jul. 18, 1939, U.S. Pat. No. 3,208,158, issued Sep. 28, 1965, U.S. Pat. No. 4,221,058, issued Sep. 9, 1980, and U.S. Pat. No. 4,413,427, issued Nov. 8, 1983.
U.S. Pat. No. 1,894,172 discloses an apparatus for determining the moisture content of gases and is adapted for determining the moisture content of a flowing current of gas. Means is provided for maintaining the gas at a substantially uniform temperature during testing.
U.S. Pat. No. 2,316,624 discloses an apparatus for determining moisture content in gases. In addition, moisture content of a gas is indicated as it flows along in a continuous stream.
U.S. Pat. No. 3,037,375 discloses a continuous vapor pressure apparatus for determining the vapor pressure of a liquid or mixture of liquids having different boiling points. A saturation chamber saturates the liquid with a gas in order to permit the gas and the liquid to attain an equilibrium condition as rapidly as possible. Saturation temperature will be determined by the liquid being tested and the specific test conditions.
U.S. Pat. No. 3,532,270 is directed to partial pressure low-level humidity generator. A solution is provided to the problem of adding extremely small and controlled quantities of moisture to dry air which differs fundamentally from the usual practice of saturating completely all or some divided portion of the air sample prior to a final operation. A sample source is supplied to a water bath and a makeup water source and temperature readouts are provided.
U.S. Pat. No. 3,265,301 is directed to an absolute humidity control device wherein a continuous sample of air is sensed from a high temperature moisture level space and is cooled to condense the moisture in the air, and the amount of heat removed to condense the moisture provides a continuous indication of the amount of moisture in a predetermined amount of air.
U.S. Pat. No. 3,665,748 discloses a calibrator moisture analyzer. The invention may be used when it is desired to produce a mixed fluid stream which consists of two fluids and which contains known concentrations of one of the fluids in the other fluid. Saturation of one fluid with the second fluid is required and means are provided for holding the saturating means at some constant temperature.
U.S. Pat. No. 4,507,875 is directed to a device for determining the concentration of condensable vapor in a flowing gas stream. The apparatus continuously determines the concentration of condensable gases or vapors in a flowing gas stream by removing a sample of the gas and conveying the sample at a constant, known volumetric flow through the conduits and to a gas condenser and flowmeter.
U.S. Pat. No. 4,597,285 is directed to a humidity control device which includes a fluidic oscillator through which a sample of gas is passed. The system is useful where the moisture content is large and there is a small difference between the molecular weight of water and the average molecular weight of the components of the gas vapor in the system.
U.S. Pat. No. 4,809,537 is directed to a system of continuously monitoring a wet bulb temperature in a flue gas. Samples of the flue gas are filtered and reheated to substantially the same temperature as the gas in the flue. The temperature of the reheated sample is measured with a sensor surrounded by a liquid absorbent wick which is immersed in a liquid maintained at a substantially constant level in a reservoir.
U.S. Pat. Nos. 2,166,379, 3,208,158, 4,221,058 and 4,413,427 all relate generally to humidity controlling devices associated with dryers.
An article entitled "A Continuous Analyser for Steam/Gas Ratio in Ammonia Plants" (authors T. S. Viswanathan and M. S. Rajappa) appeared in the March, 1983 edition of the magazine FERTILISER NEWS. The article discloses the use of an adiabatic saturator to determine the steam/gas ratio in the synthesis-gas producing section of an ammonia plant.